This invention relates in general to refractory products and processes and more particularly to a high density, low porosity refractory product made substantially from very fine materials and to a process for making the same.
In refractory shapes, particularly refractory brick, it is desirable to have high density and low porosity, and all processes for producing these refractories strive for that end, with the exception of insulating firebrick which are characterized by low density and high porosity. The conventional dry press process produces a brick which is suitable for many refractory purposes. Nevertheless, conventional dry pressed brick possess a lower than desired density and a higher than desired porosity. Conventional dry pressed brick also have a relatively high number of large pores, which is undesirable.
More specifically, the manufacture of refractory brick by the dry press process normally involves using a size graded mix of coarse, intermediate and fine sized particles. A typical screen analysis of such a brick mix is:
______________________________________ Pass 4 mesh and retained on 10 mesh 25% Pass 10 mesh and retained on 28 mesh 20% Pass 28 mesh and retained on 65 mesh 10% Pass 65 mesh 45% ______________________________________
The use of such a size graded mixture allows the brick to be pressed to a reasonably high density without forming laminations in the brick perpendicular to the direction of pressing. If too high a proportion of the mixture is composed of the fine fraction (-65 mesh), there is a tendency for the mixture to contain entrapped air during pressing. This entrapped air is compressed during pressing, and, when the pressure is released, the air expands and causes highly undesirable laminations in the brick. These laminations tend to be further accentuated when the bricks are subjected to high temperatures during the firing process.
While the use of the size graded mixture allows forming solid brick free of laminations, such a mixture gives relatively low density and high porosity to the brick. Using the foregoing typical size gradation, there are limits to the values of density and porosity attainable.
It is further generally recognized that in addition to the amount of pores present in a refractory brick the size of the pores is important, with the smallest size possible generally being most desirable. When the foregoing typical gradation is used for a refractory body, the pore sizes resulting cover a wide range with an appreciable proportion of the pores being of large size, i.e., on the order of 10 to 40 microns in diameter.
It is possible to improve the density, porosity, and size of pores developed in a refractory body by using a fine grained mixture. Herron, U.S. Pat. No. 3,522,065 discloses the use of finely milled raw materials to improve properties in direct bonded magnesia-chrome refractories. The patent does not reveal a method of manufacturing refractory pieces of a useful commercial size from the fine raw materials without cracking. Cracking may be minimized by means of isostatic forming. Following is a procedure typical of one form of an isostatic process:
(a) A mixture of finely divided raw materials is vibration packed into a rubber mold which is sealed and contained in a perforated metal container; PA1 (b) The mold and container are placed into a high pressure vessel; PA1 (c) The refractory material in the mold is exposed to a high pressure in the vessel by means of a fluid, such as oil or water, which is pumped into the vessel to exert pressures up to 50,000 psi on the mold; and PA1 (d) The refractory material is formed into an object which assumes the shape of the mold.
By exerting pressure on the body in this fashion, the air normally entrapped in the fine grained body is removed before it can cause pressure laminations to form. Also, the pressure is exerted equally on all surfaces of the shape or refractory body. This method of pressure application produces a shape practically free of the stresses normally formed in a body which is exposed to forming pressures exerted primarily in one direction. Following forming, it is sometimes necessary to dress the shape in the green state before it is exposed to high temperature firing.
The isostatic forming process necessitates the use of equipment and processes not common to the refractories industry, e.g., isostatic pressure application, green finishing, etc. This, in turn, leads to the need for a separate plant to manufacture products by this process. Also, the isostatic process is a much more expensive manufacturing procedure than the conventional dry press process. This is particularly true when conventional size pieces or shapes are being manufactured. However, the isostatic process is ideal for forming large pieces weighing several hundreds of pounds and having the highly desirable properties of high density, uniformity, low porosity, and high strength.
Thus, while isostatic forming allows the manufacture of high strength, high density, and low porosity refractories from fine grain bodies by pressing, the isostatic process is characterized by the need for new processing plants and high manufacturing costs.
Another method of making a magnesia-chrome refractory from finely milled dead burned magnesite and chrome ore was described by I. S. Kainarskii et al. (OGNEUPORY, No. 1, pp 32-37, January 1972) where the finely milled raw materials were mixed, moistened with sulfite lye and then pressed into briquettes. After drying the briquettes were crushed and screened into fractions. These fractions were blended and moistened with sulfite lye prior to pressing into brick. It was found, however, that green brick prepared in this manner developed cracks during drying as a result of hydration of the fine periclase. To eliminate the cracking it was necessary to prefire the briquettes at 1100.degree.-1300.degree. C. The necessity of costly prefiring the briquette is a disadvantage of the Kainarskii process. Applicant avoids the problem of cracking of the final shape by tempering the fine grains with a chemical bonding solution. Also, the use of high purity hydration resistant periclase also aids in avoiding this problem when an MgO basic brick is being made. The high purity hydration resistant periclase contains little CaO and SiO.sub.2 and shows little hydration when tested by ASTM T544.
Singer in an article entitled "Humid--Pressing and Dry-Pressing of Porcelain and Steatite", The British Clayworker, Vol. 61, p. 242-246, November 1952, forms "bricks" from finely ground material and regranulates these to form particles large enough to exactly fill a die in a dry pressing operation.
While other examples of the preparation and use of dense granules of various materials as a precursor to dry pressing ceramics can be found in technical literature, the art of using dense granules without costly prefiring in the manufacture of dry pressed dense commercial basic refractories has not been described.